One approach to drag reduction in aircraft, which can lead to significant fuel savings, includes modification of the laminar airflow across an airfoil surface by making it porous and applying a controlled vacuum from within the airfoil body. Airflow through the panels which form the airfoil surface may occur through thousands or millions of tiny holes, each only a few thousandths of an inch in diameter, which may be “drilled” in the panel using a high-power pulsed laser beam. The holes may be distributed around the surface of the panel in a pattern with a density that is determined by the aerodynamic design.
In order to achieve the expected drag reduction of an airfoil surface, it may be necessary to ensure that the manufacture of porous airfoil surfaces be accomplished within specific tolerances, expressed in terms of pressure drop (Pd) across the panel, and that the relative designed geometry of regions of varying Pd be rigidly controlled to this end. However, the available methods of measuring pressure drop through the panel may be cumbersome, slow, and require painstaking care to achieve the required accuracy.
Optical porosity seeks to establish a correlation between Pd and the amount of light that can be transmitted through an hole or holes in a panel using a backlight and some form of light sensor. In the paper “Light transmission control technique and correlation with pressure loss characteristics of perforated panels for Hybrid Laminar Flow Applications” presented by B. Paluch at the Proceedings of the CRAS/DragNet European Drag Reduction Conference, 19-21 Jun. 2000, in Potsdam, Germany, it was established that there is a high correlation (0.956 in the test configuration described) between total air flow through a test panel and the optical power transmitted through its perforations from a halogen point light source.
Results from shape analysis experimentation suggest that the feature of an hole in a panel that is most strongly correlated with pressure drop through the panel is simply the area of the hole. Research into laminar flow reduction suggests that the geometry of distribution of the holes within a panel may be an important factor which determines drag reduction performance, and the locations of completely plugged holes in the panel may have a significant effect.
Therefore, an expeditious and efficient method of inspecting large areas of a perforated panel which utilizes optical porosity of the panel to determine pressure drop through the panel is needed.